1. Field of the Invention
The present invention relates to a reduction-type projection printing apparatus used in manufacturing, for example, a semiconductor device, a liquid crystal device, a dielectric device, a ferroelectric device, a magnetic device, and a superconductor device, and in particular, to a spatial filter used for improving resolution and depth of focus in the reduction-type projection printing apparatus.
2. Description of the Background Art
With an increase in degree of integration of a semiconductor device in recent years, implementation of a circuit having a finer pattern is required and further improvement in photolithography technique is desired. Attempts have been made to improve resolution limit R and depth of focus DOF in a reduction-type projection printing apparatus called an optical stepper which is used in photolithography. Use of light having shorter wavelength has been tried, and at the same time, associated technology for an optical lens system and a photoresist, for example, has been developed in order to improve resolution limit R of the projection printing apparatus. G-line (wavelength of 436 nm) and i-line (wavelength of 365 nm) which are bright lines of a high pressure mercury lamp, and a deep UV region (wavelength of 248 nm) included in KrF excimer laser have been used as light of short wavelength.
However, with rapid increase in degree of integration, in particular, of a semiconductor device, a line width in a circuit is desired to be at most 0.3 .mu.m, and further, equal to or less than a light wavelength.
Resolution limit R and depth of focus DOF in the projection printing apparatus are generally given by the following Rayleigh's equations (1) and (2): EQU R=k.sub.1 .multidot..lambda./NA (1) EQU DOF=k.sub.2 .multidot..lambda./(NA).sup.2 ( 2)
where k.sub.1 and k.sub.2 are constants which are determined depending on a process, .lambda. is a light wavelength and NA is numerical aperture of the optical system.
Accordingly, it can be seen from the equation (1) that an increase in the numerical aperture NA of the optical system is necessary in order to improve resolution limit R without changing light wavelength. As can be seen from the equation (2), however, increase in the numerical aperture NA of the optical system causes reduction in depth of focus DOF which is another important characteristic in photolithography. That is, the numerical aperture NA must be optimized in order to obtain a preferred combination of resolution limit R and depth of focus DOF.
It is technically difficult to increase the numerical aperture NA, and the maximum numerical aperture NA generally achieved at present is about 0.6. In a generally used quartz-type lens material, compensation of chromatic aberration in light of short wavelength is difficult, and absorption of light by the lens material is increased, which leads to distortion of lens caused by heat resulting from the light absorption.
In recent years, some proposals have been made to improve resolution and depth of focus without the need to both reduce light wavelength .lambda. and increase the numerical aperture NA in the optical system.
FIG. 1 schematically shows a photomask disclosed in Japanese Patent Laying-Open No. 57-62052 (Japanese Application No. 55-136483). This photomask 1A includes a plurality of transparent portions 1a and a light-shielding pattern 2. The plurality of transparent portions 1a correspond to a plurality of lines which are arranged parallel to each other at a constant pitch. A .lambda./2 plate 3a which functions as a phase shifter is provided at every other transparent portion 1a. When such a photomask and coherent illumination light are used in the projection printing apparatus, phase of amplitude distribution of light passing through phase shifter 3a is reverse to that of light passing through adjacent transparent portion 1a as shown by solid curve in FIG. 1. Accordingly, in an image forming plane, light intensity distribution as shown by dotted curve in FIG. 1 can be obtained by light interference. That is, width of the light intensity distribution is smaller than that of the amplitude distribution for one projected line, resulting in improvement in resolution of a projected image.
FIG. 2 shows a cross section of a photomask disclosed in Japanese Patent Laying-Open No. (Japanese Application No. 60-206664). Photomask 1B includes a light-shielding pattern 2 formed on a transparent substrate 1. Light-shielding pattern 2 includes an isolated line-shaped aperture 5a having a width close to the resolution limit. Light-shielding pattern 2 further includes a plurality of line-shaped apertures 7 which are arranged at a constant pitch. When such a mask is used in a projection printing apparatus, intensity of light passing through the isolated aperture 5a on an image forming plane tends to be lower than that of light passing through one of the grouped apertures 7. Consequently, it is desired, for example, to increase intensity (dose) of radiation light in order to cause sufficient photochemical reaction in a resist on a semiconductor wafer by light passing through isolated aperture 5a. If dose of radiation light is increased, however, not only intensity of light passing through each of grouped apertures 7 but also width of its intensity distribution thereof is increased. Accordingly, a plurality of light-shielding dots 6a having dimension equal to or less than resolution limit are provided in each of apertures 7 in order to prevent undesired increase in intensity of light passing through each of grouped apertures 7. In addition, a phase shift layer 8 is provided in every other aperture 7 as in the photomask of FIG. 1 so as not to widen intensity distribution of light passing through each of grouped apertures 7.
FIG. 3 shows a reduction-type projection printing apparatus disclosed in Japanese Patent Laying-Open No. 4-101148 (Japanese Application No. 2-218030). The reduction-type projection printing apparatus includes a spatial filter 9, a condenser lens 10, a photomask 11 having a light-shielding pattern 12, another spatial filter 15 located at a pupil of the optical projection system 13, and a semiconductor wafer 17 located on an image forming plane. In such a projection printing apparatus, when photomask 11 has light-shielding pattern 12 including a plurality of parallel lines arranged at a constant pitch as shown in FIG. 4B, spatial filter 9 is located on a Fourier transform plane of pattern 12. As shown in FIG. 4A, spatial filter 9 includes apertures 9a and 9b which correspond to a Fourier transformed pattern of light-shielding pattern 12 shown in FIG. 4B. Pencils of rays L.sub.il and L.sub.ir of illumination light L.sub.i which pass through two apertures 9a and 9b, respectively, are diffracted by light-shielding pattern 12. Light beams L.sub.10 and L.sub.r0 shown by solid lines represent zeroth-order diffracted beams of pencils of illumination light rays L.sub.i1 and L.sub.ir, respectively, and light beams L.sub.11 and L.sub.r1 shown by broken lines represent first-order diffracted beams of pencils of illumination light rays L.sub.i1 and L.sub.ir, respectively. As shown in FIG. 4C, spatial filter 15 located at pupil 14 of the projection optical system includes a pair of apertures 15a and 15b for passing only the zeroth-order diffracted beams L.sub.10 and L.sub.r0 as well as the first-order diffracted beams L.sub.11 and L.sub.r1 therethrough. However, spatial filter 15 is not always necessary. In the reduction-type projection printing apparatus as shown in FIG. 3, resolution and depth of focus can be improved if there is a Fourier transform relation between the light-shielding pattern on the photomask and the pattern of apertures in the spatial filter.
As described above, resolution is effectively improved by reduction of light wavelength. However, with reduction of wavelength, a light source with a narrow wavelength band and a lens system with small aberration are required, and a photoresist suitable for light of short wavelength must be developed. At present, development of a light source which can emit light of shorter wavelength and of a high-performance lens system seems to be reaching the limit. In addition, development of new light source, lens system, photoresist and the like causes an increase in cost of lithography.
On the other hand, in a method of improving resolution and depth of focus by improving a photomask or by providing a spatial filter with a projection printing apparatus, it is possible to use an existing light source, lens system, photoresist and the like, which causes little increase in cost of lithography.
However, in a photomask of the prior art as shown in FIG. 1, resolution and depth of focus can be improved in regularly repeated pattern only. In some cases, a pattern which does not actually exist in a mask pattern could be formed on the image forming plane by phase inversion caused by a phase shifter 3a. Moreover, in a design for laying out a complex circuit pattern, since a number of phase shifters must be appropriately located, the design becomes more complex and, depending on circuit structure, inconsistency may occur in laying out phase shifters. In addition, in manufacturing a mask, the step of aligning phase shifters with high accuracy, which is not included in a conventional manufacturing process of a mask, is required, no matter whether a phase shifter is formed on or under the light-shielding pattern. Moreover, for defect inspection and correction of a photomask, a phase shifter formed of a transparent film which is completely different from a conventional light-shielding film, i.e. Cr film, must also be inspected and corrected. However, no practical method of inspecting and correcting such a transparent phase shifter has been provided so far.
In a photomask of the prior art as shown in FIG. 2, an attempt is made to improve resolution of an isolated small aperture 5a. However, there are problems with the photomask of FIG. 2 which are similar to those of the photomask in FIG. 1 since the photomask of FIG. 2 includes a phase shifter 8 similar to as the photomask of FIG. 1. In addition, stable formation of small light-shielding dots 6a is difficult, and if light-shielding dots 6a are formed large, a pattern of the large light-shielding dots might be formed on the image forming plane.
Although improvement in resolution and depth of focus can be achieved in the prior art shown in FIG. 3 simply by locating a spatial filter in the illumination system of the projection printing apparatus using a conventional normal mask with no phase shifter, the improvement depends on direction of pattern layout, which means the improvement can be achieved only in limited direction of pattern layout. For a pattern with a line width larger than a prescribed optimized line width, depth of focus (DOF) characteristic is significantly degraded from that obtained in the normal illumination. Also for a plurality of lines with the same line width, DOF characteristic depends on density of a line pattern, and DOF characteristic can not be improved independent of variation in pitch between lines. That is, in the spatial filter of the prior art as shown in FIG. 3, resolution and depth of focus can be improved only when the spatial filter has a particular geometric relation with a pattern on a photomask, and resolution and depth of focus cannot be improved for all the photomasks including various pattern sizes and layout conditions.